I have got a table that has a columm that has the following settings:

Field (or name): Opponent
Type: smallint(5)
Collation: blank
Attributes: UNSIGNED
Null: Yes
Default: NULL

If I do an insert query using the below $team_id_opp variable the table new row will have a value of zero (0) instead of the value of NULL.
PHP Code:
$team_id_opp    =    NULL;

INSERT INTO Games Opponent VALUES ('$team_id_opp')  // Value for the Opponent cell turns out to be "0" instead of NULL 
If I put single quotes or double quotes around NULL like below the table row still has a value of zero instead of NULL for the Opponent table cell value.
PHP Code:
$team_id_opp    =    'NULL'// Still has 0 instead of NULL
$team_id_opp    =    "NULL"// Still has 0 instead of NULL 
If I remove the $team_id_opp variable from the script altogether and do an insert query with no variable value at all it still inserts a 0 instead of the NULL value.

Question. How can I make the Opponent column cell in a new row have the default NULL value instead of a value of 0?